1. Field of the Invention
The present invention relates to a shift control system of an automatic transmission for a vehicle.
2. Description of the Prior Art
In an automatic transmission for a vehicle, the combination of a planetary gear mechanism and a plurality of engaging elements such as clutch, brake or the like which are actuated by oil pressure and the combination of engagement and disengagement of the engaging elements achieve a plurality of gear positions which respectively provide predetermined gear ratios. In case of performing shift operations between each gear, a working fluid pressure applied to the engaging elements is controlled so as to suppress shift shock at the time of engaging the engaging elements. Here, a gear ratio is represented by a ratio between an input speed and output speed of the automatic transmission
Moreover, for the driver to operate the automatic transmission, in addition to P (parking) range and N (neutral) range, as running range, there is provided xe2x80x9c1xe2x80x9d range or xe2x80x9c2xe2x80x9d range, within which a gear position is limited to the first gear or up to the second gear position, as well as D (drive) range, within which a shift operation is performed in order from the first gear position to the highest speed gear position according to driving condition. Using a selector lever, these ranges can be selected in accordance with running environments.
When the selector lever is shifted from P range or N range to D range for a starting while the vehicle is stopped, the automatic transmission engages and disengages predetermined engaging elements to establish the first gear position in response to this operation of the selector lever.
Here, since a gear ratio in the first gear position is large compared with gear positions of high speed side and a high torque is applied to an engaging element to be engaged, a working fluid pressure supplied to actuate the engaging element is set to a relatively high level.
As a result, at the time of shifting to the first gear position which is automatically determined during a shift to D range from N (neutral) range, strong shift shock may occur when the engaging element is engaged although the working fluid pressure is supplied under control.
In order to reduce the shock occurring during a shift to D range from P range or N range, for example, in an automatic transmission disclosed in Japanese Patent Application Laid-open No. 9-152024, the highest speed gear position, such as a third gear position, is temporally set at the same time as the start of preparation for a shift to the first gear position.
More specifically, by engaging an engaging element for the third gear position, to which a low torque is applied, the anxiety about shift shock caused thereby is eliminated, and simultaneously by reducing difference between this low torque and a torque applied to an engaging element to be engaged at the first gear position, shift shock at the time of shifting to the first gear position is relieved.
The engaging element is provided with friction elements on a driving side and a driven side and engages or disengages these friction elements according to the supplied oil pressure. However, even if a shift command is given to start the supply of oil pressure to the engaging element, the friction elements do not start to engage at once. An actual engagement of these friction elements is not started until an oil pressure chamber of the engaging element is filled with working fluid. This requires passing through a standby phase until a gear ratio starts to change.
In this standby phase, generally as shown in FIG. 8A, a shift command is given to temporally increase an oil pressure command value to a high level at the time t0 and then to drop this oil pressure command value to a predetermined lower level at the time t1 and thereafter to increase the oil pressure command value till the time t3 at which a gear ratio starts to change.
On the other hand, during this period, an actual value of oil pressure indicated by a broken line changes at a lower level than the oil pressure command value. At the time t2 at which the filling of the working fluid into the oil pressure chamber is completed, there occurs a surge S in the actual oil pressure, and this actual oil pressure rises to the vicinity of the oil pressure command value. In case the filling of the working fluid into the oil pressure chamber is completed while the oil pressure command value is relatively low, shock due to this surge does not exert much influence.
However, if the third gear position as the high speed gear position is temporally set when a shift operation to D range from P range or N range is performed, it becomes necessary to supply oil pressure to an engaging element to be engaged at the third gear position in addition to an engaging element to be essentially engaged at the first gear position. This makes it difficult to supply an oil of sufficient amount, and engagement of the engaging element itself for a high speed gear position may require a long time.
Thus, as shown in FIG. 8B, if the filling of the working fluid into the oil pressure chamber delays to the time t4, the oil pressure command value increases all the while, and thereby a surge S as occurs at the time of completion of filling of the working fluid increases to a level to generate shift shock.
Such a tendency is noticeable especially in the case the engine rotates at a low speed and output of an oil pump is low or in the case oil temperature is high and there is a large amount of leakage from a oil pressure supplying passage. A great shock is generated in engagement of the engaging element for a high speed gear position which is performed to relieve shift shock, and the intended object cannot be achieved.
In order to solve such a problem, it is considered to change an inclination of rise of the oil pressure command value with oil temperature. If this inclination is changed, however, a degree of change of deviation (difference) between the oil pressure command value and the actual value grows depending on times (for example, t5, t6) when the filling of the working fluid is completed. For this reason, a severe shock may occur.
On the other hand, if the oil pressure command value is always set to a low level so that deviation (difference) between the oil pressure command value and the actual value is kept small without reference to when the filling of the working fluid is completed, shift time may be elongated and the engagement of an engaging element for a high speed gear position may not be established before a shift operation to the first gear position.
The present invention has been made in view of the aforementioned problem, and it is an object to provide a shift control system capable of surely preventing the occurrence of shock in an automatic transmission wherein an engaging element for a high speed gear position is temporarily engaged when a shift operation to D range from P range or N range is performed.
In the first aspect of the present invention, therefore, a shift control system for an automatic transmission which has select position detecting means for detecting a select position of a selector lever and switches a plurality of gear positions including a first gear position when the selector lever is positioned at a running range, comprises control means for temporally engaging an engaging element to be engaged at a gear position of higher speed side than the first gear position when the select position detecting means detects that the selector lever is operated from a parking or neutral range to the running range, and the control means increases an oil pressure command value for actuating the engaging element from a start point oil pressure value after a precharge pressure with a predetermined inclination, while changing the start point oil pressure value in accordance with driving environments.
According to the above described construction, even if timing of completion of filling of the working fluid to the engaging element is fluctuated depending on the driving environments, a difference between an actual value of oil pressure and an oil pressure command value is suppressed to a small level, and the occurrence of shock due to a surge at the time of completion of the filling is prevented. Especially, since an inclination of increase of the oil pressure command value is constant, the difference between the actual value of oil pressure and the oil pressure command value is prevented from widening.
In the second aspect of the invention, the shift control system further comprises engine speed detecting means for detecting a revolution speed of an engine, and the control means lowers the start point oil pressure value when the engine speed is low.
Even if the output of an oil pump is low and the timing of completion of filling of the working fluid is delayed, the difference between the actual value and the oil pressure command value does not widen.
In the third aspect of the invention, the shift control system further comprises oil temperature detecting means for detecting an oil temperature of the engaging element, and the control means lowers the start point oil pressure value as the oil temperature increases.
Even if the timing of completion of filling of the working fluid is delayed due to the leakage of the working fluid, the difference between the actual value and the oil pressure command value does not widen.
In the forth aspect of the invention, the control means lowers the oil-pressure-command value immediately after the engaging element is engaged. By this, shifting to the first gear position can be promptly accomplished, realizing an excellent shift response.
In the fifth aspect of the invention, the shift control system further comprises turbine speed detecting means connected to a torque converter and detecting a turbine revolution speed of the torque converter, and the control means determines that the engaging element is engaged when the turbine revolution speed is lower than a predetermined value. Thereby, the aforementioned engagement state can be easily determined.
In the sixth aspect of the invention, the highest gear position is set as the gear position of higher speed side.
Since the highest gear position in which torque applied to the engaging element is especially low is set as the gear position of higher speed side, shock due to a surge can be easily suppressed.
In the seventh aspect of the invention, the shift control system further comprises throttle valve opening detecting means for detecting an opening degree of a throttle valve of the engine, and the control means terminates a control to engage the engaging element to be engaged at the higher speed side gear position when the throttle valve opening is larger than a predetermined value.
When the driver intends to rapidly accelerate by depressing an accelerator pedal, the first gear position can be promptly accomplished without passing the gear position of high speed side.
The above and further objects and features of the invention will be more fully apparent from the following description when the same is considered with reference to the accompanying drawings.